This invention relates to the production of fine stable latexes of block copolymers of vinyl aromatic hydrocarbons and conjugated dienes. More specifically, it relates to the production of such latexes which have submicron particle sizes and low surfactant to polymer ratios by using appropriate cosolvents during preparation of the latexes.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise blocks of these different monomers such as configurations which are linear, radial or star, i.e. many arms radiating from a central core. The proportion of thermoplastic blocks to elastomeric blocks and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics.
It has been found advantageous to prepare latexes of these polymers in order to obtain products that can be formulated into coatings and adhesive films having little or no organic solvents. In some cases, low levels of organic solvents are useful to control evaporation of water or to serve as coalescing aids. But the quantity of organic solvent needed to control evaporation or aid coalescence will be much less than that needed to make corresponding coatings and adhesive films from organic solvent solutions (solvent-borne coatings and adhesives). Therefore, these latexes are comprised of the polymer, surfactants, and coalescing and evaporation control solvents and water.
Preparation of such latexes is well known to those skilled in the art (see U.S. Pat. Nos. 3,360,599, 3,238,173 and 3,726,824). Latexes of polyvinyl aromatic - polydiene block copolymers are conventionally made by (1) dissolving the block copolymer in an organic solvent or solvents to form a polymer cement, (2) emulsifying the polymer cement in an aqueous solution containing one or more surfactants, and (3) stripping the emulsion of organic and excess aqueous liquids to form a stable, suitably concentrated latex. Polymer cements are often available directly from the polymerization reactor.
Block copolymer cement solutions that are to be emulsified typically contain from 5 to 30 weight percent polymer. It is advantageous for the polymer concentration in the cement to be as high as possible. Increasing the polymer concentration in the cement reduces the amount of solvent and excess water that must be removed following emulsification. It also reduces the surfactant to polymer ratio in the final latex. Surfactants are usually present such that the total concentration of surfactants ranges from about 0.5 to more than 50 parts per hundred rubber (phr), where the rubber is the block copolymer. A high surfactant concentration can negatively impact adhesive properties of coatings and adhesives. It can also render coatings and adhesives water sensitive.
The maximum concentration of a polydiene-polyvinyl aromatic block copolymer in the cement is limited by the viscosity and elasticity of the cement. For some block copolymers it is not practically possible to make stable block copolymer cement emulsions with average drop size of less than one micron from cements containing a single solvent unless the polymer concentration in the cement is less than 15 weight percent. This is especially true for functionalized block copolymers containing polar groups that may associate. It is also true for unfunctionalized block copolymers having total molecular weights greater than 25,000 and polyvinyl aromatic block molecular weights greater than 5000. Moreover, emulsification of cements containing functionalized block copolymers at concentrations greater than 10 weight percent in a single solvent frequently results in multiple emulsions with large and very disperse drop size distributions. By multiple emulsions, we mean water-in-cement-in-water emulsions wherein water droplets are dispersed in cement droplets that are, in turn, dispersed in a continuous aqueous phase. Multiple emulsions are undesirable because average particles sizes are large and water can be entrained in polymer latex particles which can negatively impact coating or adhesive properties.
Fine polymer cement emulsions are necessary in order to make fine latexes. Fine particle size latexes are advantageous because particles remain stable and dispersed with less surfactant. Particles in the range of one micron and larger tend to cream because the density of the cement is less than the density of the aqueous phase. It is also known that small (submicron) particle sizes in latexes can enhance coating and adhesive performance. Thus, it is highly advantageous to be able to produce a stable polymer cement emulsion with an average drop size of less than one micron without the formation of multiple emulsions.